Presently, liquids such as beverages are filled in paper packaging materials for sale. Packaging materials after storage of liquid are formed into various three-dimensional shapes, such as triangular pyramids, rectangular prisms, or tubular shapes. In the present specification, a packaging material after being formed is referred to as a “package”.
When producing substantially rectangular prism packages, for example, among these types of packages, surfaces of a long packaging material are overlapped with each other, first, by overlapping the lateral opposite ends of the material with each other, and then bonded to each other for formation into a tubular shape. Then, the tubular packaging material is sealed at the lower end, and filled with contents, followed by sealing the upper end. The packaging material of which both ends have been sealed is pressed to form a three-dimensional rectangular prism. In the present specification, sealing of a packaging material is hereinafter referred to as “sealing”.
Ultrasonic sealing may be used as a sealing method of packaging materials. When performing ultrasonic sealing, the sealing part of a packaging material is pressed while being sandwiched between a resonator referred to as a horn and a receiving jig referred to as an anvil. Ultrasonic sealing provides vibrational energy to the sealing part via the horn simultaneously with application of pressure, and melts the thermoplastic resin on the inner surfaces of the packaging material to fuse them together. The packaging material of which both ends have been sealed is pressed to form a three-dimensional rectangular prism.
The sealing part of the packaging material includes a portion where the lateral ends of the packaging material are overlapped with each other. In the present specification, hereinafter, the portion where surfaces of the packaging material are overlapped with each other is referred to as “overlap part”. The overlap part is thicker than the rest of the sealing part, and receives high pressure via the horn and the anvil. The anvil is provided with a recess, being aligned with the overlap part, to thereby press the sealing part with a uniform force. This type of configuration is described in, for example, PTL 1.
There is a trend of increasing the speed of ultrasonic sealing more and more, and at present, packages are manufactured at a rate of 100 to 400 packages per minute. Faster processing with ultrasonic sealing may accordingly impair the positioning accuracy between the recess of the anvil and the sealing part, causing misalignment of the overlap part with the anvil. In the occurrence of misalignment, excessive pressure is likely to be applied to the overlap part, causing poor sealing that leads to leakage of the contents.
PTL 2 describes an ultrasonic sealing apparatus including an anvil that is formed with continuous grooves in a portion used for sandwiching the sealing part, thereby preventing leakage of the contents due to misalignment of the overlap part. According to such an ultrasonic sealing apparatus, in spite of the overlap part being misaligned with the anvil, ridges defined between the grooves discontinuously press the overlap part.